Piston operated hydraulic pumps and motors have long been used in a wide variety of applications involving fluid power transmission. When operated as a pump, an input shaft is driven rotatively and fluid is then pumped from an inlet side of the unit to a discharge side. When operated as a motor, fluid under pressure is directed into the unit which then drives an output shaft. A plurality of pistons are reciprocated in cylinders, either by the turning of a shaft in the pumping mode of operation or by the fluid pressure when operating as a motor.
Piston pumps and motors are generally classified either as axial units in which pistons are disposed to operate parallel with the axis of rotation of a piston carrying rotor or as radial units in which the pistons are disposed perpendicular to the said axis.
It is the object of this invention to provide a radial unit of a superior design. It is a further object to provide advantages not found in the alternative axial units.
To clarify the advantages of my invention, it is necessary to compare similar functions of the two basic designs. When in operation, the geometric differences in the two types of units create profoundly different dynamic and hydrostatic force patterns and therefore present vastly different engineering and design considerations.
One of the problems of both of these types of units is that of introducing fluid into the cylinders of a spinning rotor. The centrifugal force generated by the rotor creates a tendency to expel fluid from the cylinders. The presently marketed units require a pressurized fluid supply to the inlet port.
Certain design problems inherent in the radial configuration are explained in my co-pending application, No. 907,576 filed Sept. 15, 1986 and entitled "Pump/Motor". The said application describes the use of hydrostatically balanced or "floating" port plates fitted to a rotor and having passages extending radially to the outer ends of the cylinders.
These two features, each having little value without the other, combine to solve two important problems inherent in the radial design. The first, that of providing a sliding seal between the stationary port plate and the revolving rotor, which seal remains in sealing contact even when subjected to extreme temperature variations, and the second, that of introducing fluid into a spinning rotor in a manner to utilize the centrifugal force of the rotor to create a suction at the inlet opening, thereby making a pressurized fluid supply to the pump unnecessary.
An analysis of this combination, as well as other design features of the unit described in my co-pending application, has revealed certain function and design problems which are remedied in the present invention.
I have found that the advantage gained by introducing fluid at an inward location is at least partially lost when the fluid is forced back through the radial passages against centrifugal force to the outlet port.
This analysis has also revealed another design problem created by the aforementioned combination. Considerable force is exerted against the rotor by the telescopic piston devices used to hold the port plate against the rotor when subjected to the high pressures normally generated by this type of unit. The magnitude of this thrust force requires that very substantial bearings be used in journaling the rotor in the housing.
I have found that by placing a second port plate on the rotor opposite the first port plate, the equal and opposite forces of the two will alleviate the thrust stresses created by the single port plate. Further, if this second port plate is located near the outer periphery of the rotor, fluid can be expelled at the outer parts of the unit and need not be forced back to the inboard ports.
This single addition solves these two problems found in my co-pending application. Further analysis of the unit as described in my pending application has produced a finding that, if not constrained, the forces of the pistons against the cam while the unit is working, will be of sufficient magnitude to displace the cam from a pre-selected position. The force pattern of the pistons against the cam alternates from one direction to the opposite direction as the pistons move around the cam.
The present invention provides a means for connecting the cam to the housing by linkage to prevent undesired movement of the cam position and further provides a means of adjusting the said linkage to effectively adjust the stroke of the pistons and therefore the volume of fluid displaced by the unit.
The alternating forces of the pistons acting against the cam provide the force needed to position the cam. The said linkage is needed therefore to simply constrain against undesired movements of the cam and to provide a means of selecting the desired piston stroke.
This phenomenon, that of alternating forces of pistons against the cam, found in the radial unit, is of great significance when contrasted to the alternative axial units which require a power assist feature, in the larger units, to aid the operator in controlling the stroke of the pistons; ergo, the displacement of the unit. The tendency of rotating objects is to move in a circular path. In the axial units, this tendency of the pistons to move in a circular path creates a resistance which must be overcome in order to achieve piston stroke. In the larger units operating at high speeds, this natural tendency to follow a true circular path is quite substantial.
The need for auxiliary power to effect the changes in the volumetric displacement and the need for a pressurized fluid supply to the inlet port required in the alternative axial units are two important considerations when contrasted with the present invention which requires no auxiliary power.
Further analysis of the unit as described in my pending application has revealed several design deficiencies in respect to to excessive stresses on various component which will result in excessive friction and wear.
It is an object of this invention to provide a radial piston fluid pump/motor in which all of the undesirable characteristics found in the unit described in my pending application are satisfactorily resolved.
It is a further object of this invention to provide a means of controlling the position of the cam block, as described in my co-pending application, thereby effecting a precise control of the volume of fluid displaced by the unit.
It is a further object of this invention to provide a radial piston fluid pump/motor having a minimum of stress forces acting on all moving parts.
It is a further purpose of this invention to provide a more compact variable displacement fluid pump motor than is available in today's market.
It is a further purpose of this invention to provide a fluid pump/motor that is easily assembled and repaired and, further, convertible from a fixed volume unit to a variable volume unit with minimal effort.